Processes for the grafting of maleic anhydride on olefinic polymers, also of the elastomeric type, have been widely described in the literature, for instance in U.S. Pat. No. 4,010,223 and in the "Journal for Polymer Chem. Ed., vol. 22, 1984--page 1335.
The grafting techniques, used for the purpose, differ from one another by the physical state in which the polymer to be grafted is used, wherefore one may have processes of the so-called bulk type, or in solution or in suspension.
In the first kind of process, the polymer and the unsaturated polymer are mixed together in the presence of free radical-generating compounds, at a temperature (in general comprised between 150.degree. C. and 180.degree. C.), which render the material workable in extruders, mixers, etc. However simple, this method is substantially little advantageous due to the difficulty of controlling and reproducing the operational conditions, especially when the olefinic polymer is rubbery.
The method in solution operates with the polymer, the monomer and the radicalic starter, all dissolved in a solvent, generally an aromatic hydrocarbon, at temperatures around 100.degree. C. This method offers various different advantages, more particularly that of an effective thermal control; however, the method has the drawback of requiring low concentrations of polymer and thus, to have to separate and to recycle great quantities of solvent.
The grafting in suspension is achieved by suspending in a dispersing means, in general water, the granulated polymer, possibly blown with solvents, the monomer, the starter and the surfactant, and by then letting the reaction mixture react at temperatures comprised between 60.degree. C. and 100.degree. C.
The suspension method offers the advantages of an effective thermal control of the reaction, the possibility of operating at high concentrations of polymer, and of separating the grafted polymer by means of a simple filtering or centrifuging.
Such a method, however, is not suited for the grafting of monomers that are easily hydrolizeable in water, such as for instance maleic anhydride. In this case, in fact, the monomer that would be grafted would be maleic acid instead of maleic anhydride, and this would require further treatment at a high temperature of the grafted polymer, to eliminate the water and reform the anhydride.